1. Field of the Invention
The present invention relates to a method and apparatus to measure convective, conductive, radiant, and evaporative heat flow. More specifically, in a preferred embodiment, the invention is used to estimate total heat loss from a human body or other living subject by measuring heat flow from only one or several portions of the body, each of which is assumed to be representative of heat loss over that particular region of the body. From this measurement of total heat flow, a calculation of caloric expenditure can be made.
2. Background of the Invention
The determination of caloric expenditure is an important component of any weight control or fitness program. The number of calories burned is generally estimated through the use of tabulated values for a given activity or by the use of workload measurements on exercise equipment such as treadmills or bikes. Neither, however, is particularly reliable. The tables are generally only average rates for a 70 kg individual performing each activity in some arbitrary, average manner. Certainly not very reflective of any given individual""s caloric expenditures, the tables may vary as much as 50% from actual caloric expenditures. Exercise equipment having calorie calculators makes similar errors, and such equipment fails to provide any indication of total caloric expenditure for the day.
A more reliable approach would be to actually monitor the caloric expenditure. The body""s metabolic xe2x80x9cenginesxe2x80x9d generate significant amounts of heat; at rest this heat generation is equivalent to that of a 100 watt light bulb. In the human body""s attempt to maintain a body temperature of 98.0xc2x0 F., it controls heat loss to the environment by regulating blood flow to the body surface. At rest, blood flow to the skin is restricted and the surface of the skin may be as much as 20xc2x0 F. cooler than the body core. This results in a lower flux of heat to the environment. With exercise, however, the excess heat generated by physical exertion (approximately 80% of the energy needed to contract human muscles is wasted as heat) must be dumped to the environment to maintain constant body temperature. Blood flow is diverted to the skin, raising its temperature and the rate at which heat is dumped to the environment is increased.
As a homoiotherm, the body maintains a nearly constant internal body temperature by balancing the generation of heat by its metabolic process with controlled loss of heat through an orchestration of evaporative, convective, radiant, and conductive heat loss mechanisms. At rest in normal room temperature conditions, the body can utilize convective and radiant heat loss (with minor conductive heat loss contributions as well) to regulate body temperature, primarily by control of blood flow to the skin surfaces. If an individual is exercising or is in ambient temperatures above 35xc2x0 C., the convective and radiant heat loss is inadequate to control internal temperature and the body begins to utilize evaporative heat loss. Evaporation, both that which occurs insensibly (i.e. without obvious sweating) and sensibly (i.e. with obvious sweating) can provide several fold greater heat loss than the other two mechanisms combined.
Heat flow can be accurately measured with a whole body calorimeter. This device is a chamber in which the subject is placed and the total heat given off by the subject""s body can be captured and measured. The disadvantages of a whole body calorimeter are that it is expensive, relatively immobile, and the actions and motions of the subject are limited to the space within the chamber. See W. H. Close, M. J. Dauncey, and D. L. Ingram (1980), xe2x80x9cHeat loss from humans measured with a direct calorimeter and heat-flow metersxe2x80x9d, Br. J. Nutr. 43, 87, pp 87-93.
In order to overcome the disadvantages of the whole body calorimeter, a sampling technique using heat flow sensors has been developed to estimate the total heat loss from a subject by measuring heat loss on only a few selected locations on the subject""s skin surface. Each measured value is multiplied by a xe2x80x9cweighting co-efficientxe2x80x9d in order to estimate the heat loss for that particular region of the subject""s body. The sum of all regional heat loss components is the estimate of the total heat loss. One system of xe2x80x9cweighting coefficientsxe2x80x9d has been developed by Hardy and DuBois. See Archives of Internal Medicine, Vol. 17, No. 6, pp. 863-871 (1916).
Traditional heat flow sensors are generally based on the measurement of the temperature differential that occurs across a material due to the thermal resistance of that material. In order for the sensor to accurately measure the heat flow, it must not add a significant insulating layer and it must lose heat from its surface in the same manner as the surface on which it is placed. Certain available heat flow sensors perform well on inanimate objects such as walls, doors, boilers, and pipes, where convective, radiant, and conductive heat loss mechanisms predominate. Such heat flow sensors are, however, inadequate for measuring heat loss from the human body, where evaporative heat loss may be significant.
Some current heat flow sensors, such as that produced by RdF, are unable to reliably include the component of evaporative heat loss from the body as part of its output signal. This results in an under estimation of heat loss for two main reasons: 1) such sensors actually occlude the surface of the skin, preventing evaporation, and therefore, any moisture that does move from under the sensor evaporates from the skin surface adjacent to the sensor and not from the sensor surface itself; and 2) when used to monitor body heat loss, as the evaporative heat loss increases from the skin surface, thereby reducing the skin surface temperature, these sensors actually show a decreased heat flow.
A first object of the present invention is to provide a novel heat flow sensor capable of accurately measuring all components of heat loss, including evaporative heat loss.
A further and more specific object of the present invention is to provide a novel sensor design in the novel heat flow sensor which is particularly well suited to ensure that evaporative heat loss is accurately measured by enhancing the migration of perspiration into an appropriate part of the sensor.
To achieve the above and other objects, the present invention discloses a novel method and apparatus for determining caloric expenditure by measuring all components of heat flow. It is small, portable, relatively inexpensive, and can be worn on the subject""s body with no significant limitation on motion or mobility. The present invention utilizes a modified heat flow sensor element that is superior to heat flow sensors currently used, which fail to measure evaporative heat loss. Currently, only devices such as whole body calorimeters are capable of measuring all components of heat loss. As previously stated, these devices are large, expensive, relatively immobile, and limit the activity of the subject.
In order to improve the measurement of evaporative heat loss, the novel heat flow sensor element of the present invention can take on a specific construction such that the migration of evaporative fluid towards a center of the sensor element is enhanced. That is achieved in the present invention by inducing electroendosmosis by appropriately placing and biasing electrodes in the sensor element to enhance the migration of the evaporative fluid toward an active region of the sensor.
As a further way to improve the measurement of evaporative heat loss, the novel heat flow sensor of the present invention can take on a specific construction such that the active sensing elements are thermocouples which are placed at an edge of a sensor, so that a distance that perspiration has to migrate to an appropriate sensing position is reduced.
As additional features to enhance measuring evaporative heat loss with a heat flow sensor element, the present invention can optionally include a design of a sensor element with an overlay material which allows the evaporating fluid to migrate from the monitored surface (i.e., skin) to the ambient air side of the heat flow sensor element and subsequently to evaporate from the surface of the heat flow sensor element. To further facilitate the measurement of evaporative heat loss, the present invention can optionally create a substantially uniform temperature over the ambient air surface of the heat flow sensor and the surrounding measured surface. This can be accomplished using a thermally conductive layer which is placed over the ambient air surface of the heat flow sensor element and overlapped onto the measured surface.